HLA-DQA1 and DPB1 alleles were examined in relation to autoimmune thyroid disease (AITD) in the Japanese type 1 diabetic patients. The subjects consisted of 14 type 1 diabetic patients with Graves' disease, 12 patients with Hashimoto's thyroiditis and 32 type 1 diabetic patients without AITD. Comparisons were made with 35 normal controls. Among the type 1 diabetic patients with Graves' disease, the age at onset of diabetes was 31.8 +/- 14.6 years old, which was later than that of those without AITD (P < 0.01). DR9 was increased (57.1% vs. 25.9%, P < 0.05, RR: 3.85, chi 2:4.36) in the patients with Graves' disease. DQA1*0301 was increased and DQA1*0103 was decreased in the patients with Graves' disease and those without AITD. HLA-DPB1*0501 was increased (92.9% vs. 54.3%, P < 0.05, RR: 11.0, chi 2:6.57) in the patients with Graves' disease. These findings suggest the existence of a Graves' complicated subgroup characterized by the increasing association of DPB1*0501 and late onset of diabetes in Japanese type 1 diabetic patients. There exists a heterogeneity in Japanese type 1 diabetes.